what is a thermal overcurrent circuit breaker?

[sealegsjim]

I decided to replace the two 'rocker switches' which supply shore power to my water heater and built in battery charger as they are not illuminated and it is difficult to see at a glance whether they are on or off. It turned out that they were not rocker switches but thermal overcurrent circuit breakers and they cost about £20 each with p&p. Does anyone know their exact purpose? I have tried googling but haven't found the answer. Also does anyone know if I could wire in an indicator neon/LED and if so how? Thanks

 

[VicS]

They provide over current protection. They have been used in place of fuses. The advantage over a fuse is that you can simply switch 'em back on once the fault is cleared.

No reason why you should not wire a neon indicator to the output side in parallel with the load

But you are working on mains voltage equipment. It would be advisable to get the assistance of someone with good knowledge or experience or preferably qualified.

 

[sealegsjim]

Thanks for that very speedy reply but I am still not sure where the 'thermal' part of the description comes from

 

[pampas]

two types ,thermal and magnetic, yours has a bimetal strip to activate.

 

[Norman_E]

Thanks for that very speedy reply but I am still not sure where the 'thermal' part of the description comes from

Circuit breakers described as "thermal" are normally in fact both electromagnetic and thermal. They have an electromagnet where the pulling force increases with current, pulling the switch "off" instantly when there is a sudden large overcurrent situation, and a bi-metal strip which trips the breaker if heated beyond a certain point. That deals with a situation where there is a small over current over a longer period.

 

[Silent Lady]

Not like a fuse

If you had a fuse in the circuit then you would have to have one large enough to withstand switching on all the items in one go including fridge motors etc.

The idea behind the thermal fuse is that it can cope with the fluctuations and high loads but will only trip if you have a permanent problem. They usually work by starting to get warm if you draw to much current for to long and then trip. Leave it to cool and it will work again.

It also means that you can work out whats at fault by switching things on. Wait 10 seconds and try the next till you find the fault. Also saves having a supply of fuses at night in a dark marina.

 

[VicS]

Thanks for that very speedy reply but I am still not sure where the 'thermal' part of the description comes from

Just means that they rely on heat produced within them to trip them.

Dropped back really to suggest that it might be possible to find similar devices with built in neon indicators

Or something like these http://www.rapidonline.com/Electron...cators/Push-fit-neon-indicators/61840/kw/neon

 

[halcyon]

Thanks for that very speedy reply but I am still not sure where the 'thermal' part of the description comes from

It is a blade of two dissimilar metals welded together, as a current passes through it, it get's hot, causing one side to expand at a faster rate than the other.

You put a pre-bend in the blade, once the expansion on the inner face exceeds the pre-bend, it flicks the blade the other way, opening the contacts and stopping current flow.

A manual reset thermal breaker now locks the contacts open till you manually reset it, auto reset on cooling just flick back to start possition and start conducting again.

Brian

 

[sealegsjim]

Thank you everyone for your input. There's a wealth of expertise in this forum. I have decided to keep the original breakers and wire in neon indicator lights which I can get at Maplins for about £1.50 each

 

[VicS]

Thank you everyone for your input. There's a wealth of expertise in this forum. I have decided to keep the original breakers and wire in neon indicator lights which I can get at Maplins for about £1.50 each

If you can walk into a Maplin and get what you want i'd not disagree but Rapid Electronics are good and established themselves supplying schools. I think the physics department where i worked for a while completely ditched RS in favour of them.

 

[GandT]

thermal CBs use bi-metalic strips to track the current. There one big advantage is that they are self compensating which protects the insulation. ie if the ambient temp rises (the tropics) then the CB is part way towards tripping amd will protect the insullation before it breaks down.